1. Field of the Invention
The present invention relates to a semiconductor laser, a semiconductor optical amplifier, and a production method thereof.
2. Description of the Related Art
Nowadays, the opto-electronic technology is used in various fields, e.g. information I/O systems such as compact disc applications, and optical telecommunication systems using optical fibers. As devices for supporting the opto-electronic technology, various semiconductor lasers have been developed. For example, semiconductor lasers operating at near infrared or visible light band have been developed for use with compact discs, and semiconductor lasers of long wavelength band have been developed for use with optical telecommunication systems.
As a type of the semiconductor laser, there is a waveguide type semiconductor laser. In general, when a waveguide type semiconductor laser is used for information transmission or as an optical fiber amplification excitation source, its waveguide is designed so as to satisfy a single mode condition or a quasi-mode condition.
In general, when a laser beam is used in multi mode, there arises a problem with the multi mode dispersion. When a signal light is coupled to an optical waveguide such as an optical fiber or to a lens, there arises a difficulty in coupling a signal light effectively. These problems can be eliminated by designing the waveguide as mentioned below.
In order to satisfy the aforementioned single mode condition, various semiconductor lasers have been developed. (For example, Japanese Patent Publication 63-293989 discloses such a semiconductor laser element.) However, the configuration designed to satisfy the single mode condition has restricted the laser characteristics.
For example, a width and a thickness of an active layer in a semiconductor laser have been limited within a predetermined range according to the single mode condition. Especially, in order to improve a gain saturation level, it is advantageous to enlarge the waveguide width. However, this is limited to a certain value because of the aforementioned single mode condition. Thus, semiconductor laser output power has been limited technically.
An example of eliminating such limitation by the single mode condition is disclosed in L. J. Mawst et al. xe2x80x9cPhase-locked laser diode arrayxe2x80x9d, Applied Physics Letters Vol. 60, No. 6, 1992, pp. 668-670. In this phase-locked laser diode array, 20 semiconductor lasers of single mode are arranged at a predetermined interval in a direction vertical to the light waveguide, so that the semiconductor lasers produce resonance, thus enabling to finally obtain a high single mode output.
However, this phase-locked laser diode array has a complicated configuration, which results in a low production yield.
Moreover, when producing the phase-locked laser diode array so as to satisfy the resonance condition, the tolerance is very small and it is difficult to produce identical arrays.
Furthermore, in addition to the active layer gain saturation, COD (catastrophic optical damage) and spatial hole burning are known as phenomena constricting the semiconductor laser output. In general, in order to improve the COD level, it is preferable to enlarge the waveguide width so as to reduce the light density per unit area of the cleaved facet or end.
However, the waveguide width is under restriction of the single mode condition as is mentioned above. Furthermore, in order to suppress the spatial hole burning, in general, it is preferable to reduce the waveguide width. Thus, in order to obtain a high output in the conventional semiconductor laser, it is necessary to perform designing, satisfying the single mode condition and considering the COD level and the spatial hole burning which are in a trade off relationship with the single mode condition. Accordingly, it has been tremendously difficult to obtain a high output of a semiconductor laser.
As a method to improve the aforementioned COD level, 10 a semiconductor laser using a flare type waveguide is suggested by M. Sagawa et al., Electronics Letters, Vol. 32, No. 24, 1996, pp. 2277-2279. In this semiconductor laser of the flare type, the width of the waveguide is gradually increased from backward end toward the forward end, so as to reduce the light density at the light emitting surface while keeping the single mode light emission, thus enabling to improve the COD level and to obtain a high output.
However, the light propagating mode greatly depends on the flare configuration. The flare configuration cannot be easily reproduced for obtaining the single mode output. For example, if, during a production, a slight pattern error or waveguide unevenness is caused, it is difficult to obtain the lateral single mode output as is designed with a high yield, and multi mode oscillation may occur. Alternatively, the spatial hole burning, i.e., multi mode oscillation due to current flow-in may occur.
Moreover, because the cleaved end has a flare configuration, a high accuracy is required for the cleaving position and it has been difficult to obtain identical lateral single mode output.
Furthermore, in the aforementioned semiconductor laser, in principle, mode conversion is performed along the entire propagating direction and the mode conversion loss is inevitable. That is, it is extremely difficult to obtain a highly efficient output by the aforementioned semiconductor laser.
In addition to the aforementioned document, Japanese Patent Publication 9-199782, Japanese Patent No. 2545719, and Japanese Patent No. 2723945 suggest semiconductor lasers using a waveguide of the flare configuration. However, these semiconductor lasers have the same problems as has been described above.
With respect to a semiconductor optical amplifier, a tapered configuration is implemented to improve a saturated output level. This is reported by Chih-Hsiao Chen et al, xe2x80x9cTechnical Digest of Optical Amplifier and Applicationsxe2x80x9d, MC3, 1998, pp. 39-42. In this semiconductor optical amplifier, by implementing a wide region, so as to increase the active layer area compared to the conventional single mode waveguide, thus improving the saturated output level.
However, the light output mode greatly depends on the shape of the tapered configuration. The tapered configuration cannot be easily reproduced for obtaining the single mode output. For example, if, during a production, a slight pattern error or waveguide unevenness is caused, it is difficult to obtain the lateral single mode output as is designed with a high yield, and multi mode oscillation may occur. Alternatively, the spatial hole burning, i.e., multi mode oscillation due to current flow-in may occur.
Furthermore, the aforementioned semiconductor optical amplifier has a configuration that in principle, mode conversion is performed over all the waveguide directions and mode conversion loss is inevitable. Accordingly, it is quite difficult to obtain a highly efficient output by the aforementioned semiconductor optical amplifier.
As has been described above, the conventional single mode semiconductor laser has a problem that it is difficult to obtain a high output because of a gain saturation of the active layer due to the single mode condition, the spatial hole burning, the COD level, and the like.
Another conventional example, though solving these problems, has a problem of configuration complexity. The production condition tolerance is strict and reproducibility is very low. In principle, the mode is unstable and there is a problem of spatial hole burning. The efficiency is lowered by the mode conversion loss. Moreover, it is difficult to obtain a stable lateral mode.
These problems are also present in the conventional optical amplifier.
It is therefore an object of the present invention to provide a semiconductor laser and a semiconductor optical amplifier capable of realizing a single mode output, which is one of the important characteristics of a waveguide optical device, and obtaining a high gain and high output as well as improvement of the COD level and the spatial hole burning to obtain a stable lateral mode, and not generating a mode conversion loss in principle. Another object of the present invention is to provide a production method of such a semiconductor laser and a semiconductor optical amplifier.
In order to achieve the aforementioned object, the present invention in a first embodiment provides a semiconductor laser comprising: a single mode waveguide and a first multi-mode waveguide, wherein the first multi-mode waveguide has a greater width than the single mode waveguide, the single mode waveguide provides a single mode to an oscillated light oscillated from an active light waveguide, the first multi-mode waveguide provides modes including a multi-mode to the oscillated light, and the semiconductor laser has a light output port consisting of the first multi-mode waveguide.
In another embodiment of the invention, the single mode waveguide is, for example, connected to one of the light output ends of the first multi-mode waveguide.
In yet another embodiment of the invention, a tapered waveguide is connected between the single mode waveguide and the first multi-mode waveguide.
In a further embodiment of the invention, the first multi-mode waveguide may have one-port at one side and N-ports at the other side of it (N is a positive integer), i.e., 1 XN, multi-mode interference type waveguide. For example, the first multi-mode waveguide may be a one-input one-output, i.e., 1xc3x971, multi-mode interference type waveguide.
A still further embodiment of the invention provides a semiconductor laser comprising: a single mode waveguide, a first multi-mode waveguide, and a second multi-mode waveguide, wherein the first multi-mode waveguide has a greater width than the single mode waveguide, the single mode waveguide provides a single mode to an oscillated light oscillated from an active light waveguide, the first and the second multi-mode waveguides provide modes including a multi-mode to the oscillated light, and the semiconductor laser has a light output end constituted by an end of the second multi-mode waveguide.
According to yet another embodiment of the invention, it is preferable that the second multi-mode waveguide be a secondary mode cleaved waveguide.
In an additional embodiment of the invention, the second multi-mode waveguide may be a one-input one-output, i.e., 1xc3x971, multi-mode interference type waveguide.
A further embodiment of the invention provides a semiconductor laser production method comprising: a first step of successively forming on a substrate, a buffer layer, an active layer, and a first cladding layer, a second step of removing portions of the buffer layer, the active layer, the first cladding layer, and the substrate so as to form a mesa, a third step of successively forming a first current blocking layer and a second current blocking layer around the mesa in such a way that only the first current blocking layer is in contact with the side wall of the mesa and only the second current blocking layer is exposed outside, a fourth step of successively forming a second cladding layer and a cap layer to cover the first current blocking layer and the mesa, a fifth step of forming a rear electrode and a front electrode; and a sixth step of applying a half-reflection coating to one end of the waveguide and an anti-reflecting coating to the other end after an element is cleaved.
Yet another embodiment of the present invention provides a semiconductor optical amplifier comprising: a single mode waveguide, a first multi-mode waveguide, and a reflection preventing an end formed on both ends, wherein the first multi-mode waveguide has a greater width than the single mode waveguide, the single mode waveguide provides a single mode to an amplified light amplified by an active light waveguide, the first multi-mode waveguide provides modes including multi-mode to the amplified light, and the semiconductor optical amplifier has a light output end constituted by an end of the first multi-mode waveguide.
Still yet another embodiment of the invention provides a semiconductor optical amplifier comprising: a single mode waveguide, a first multi-mode waveguide, a second multi-mode waveguide, and anti-reflection ends formed on both ends, wherein the first multi-mode waveguide has a greater width than the single mode waveguide, the single mode waveguide provides a single mode to an oscillated light oscillated from an active light waveguide, the first and the second multi-mode waveguides provide modes including a multi-mode to the oscillated light, and the semiconductor optical amplifier has a light output end constituted by an end of the second multi-mode waveguide.
By applying a reflection preventive means such as an anti-reflection coating to the both ends of the semiconductor laser, it is possible to obtain a semiconductor optical amplifier. Accordingly, the semiconductor lasers of the present invention can be directly used as the semiconductor optical amplifiers.
A further embodiment of the present invention provides a semiconductor optical amplifier production method comprising: a first step of successively forming on a substrate, a buffer layer, an active layer, and a first cladding layer, a second step of removing portions of the buffer layer, the active layer, the first cladding layer, and the substrate so as to form a mesa, a third step of successively forming a first current blocking layer and a second current blocking layer around the mesa in such a way that only the first current blocking layer is in contact with the side wall of the mesa and only the second current blocking layer is exposed outside, a fourth step of successively forming a second cladding layer and a cap layer to cover the first current blocking layer and the mesa, a fifth step of forming a rear electrode and a front electrode; and a sixth step of applying an anti-reflecting coating to the ends after an element is cleaved.